Measurement of soil water tension is useful in determining moisture content of soil, for example in farmland to determine irrigation scheduling. Instruments for measurements of soil water tension, or tensiometers, are described in Chapter 23 of "Methods of Soil Analysis" (D. K. Cassell and A. Klute 1986, American Society of Agronomy, Madison, Wis.) (ref) and generally comprise a sealed water reservoir, a solid wicking material mounted at the bottom of the reservoir in contact with the water within the reservoir and extending into the soil to be measured, and a means for measuring pressure within the reservoir. The measured pressure ranges from 0.0 to 700.0 cm water and may be measured with a manometer or with a hypodermic inserted through a septum mounted on the top of the tensiometer opposite the wick material. Commercially available tensiometers also known as ceramic cup tensiometers, have an inflexible ceramic cup as the wick. The ceramic cup is permanently bonded to the bottom of the water reservoir.
In operation, it is necessary to maintain an air tight seal against the vacuum that is created within the tensiometer water reservoir when water is drawn through the saturated wick into the unsaturated soil. It is also necessary that the water reservoir and wick resist large deflections that may be caused by the vacuum. The commercially available ceramic cup tensiometers meet these requirements by using a rigid porous ceramic material in the shape of a cup for the wick, and by permanently bonding the ceramic cup to the water reservoir.